Various methods have been investigated for the synthesis of dienes. These include:
Acid-catalyzed dehydration of 1, 2-, 1,3-, or 1,4-diols. Note for example, Noller, Chemistry of Organic Compounds, W. B. Saunders Company, Philadelphia and London, 3rd Edition, 1965, page 778.
Coupling of vinyl halides with various vinyl metal reagents catalyzed by transition metal compounds. See for example Dang et al., Tetrahedron Letters, 1978, pages 191-194 and references cited therein.
Formation from allylic alcohols via oxirane and enediol. Tanaka et. al., J. Am. Chem. Soc., 1975, 97, pages 3252-3254.
Palladium catalyzed elimination reactions using allylic acetates and allylic phenyl ethers. Tsuji et al., Tetrahedron Letters, 1978, No. 24, pages 2075-2078.
Base-catalyzed thermal decomposition of methohydroxides and methodeutero-oxides of various 5-N,N-dimethylamino-pent-1-enes. Cocker et al., J. Chem. Soc., Perkins Trans., 1984, I, pages 2245-2254.
U. S. Pat. No. 4,467,118, to Chalk et al., describes conversion of tertiary allylic amines to dienes via a zero valent palladium phosphine complex and a weak acid.
In Journal f. prakt. Chemie, Band 326, Heft 1, 1984, pages 12-22, Beger et al. report that a product containing 1,3-alkadiene was produced from the reaction of 1,2-dihaloalkanes and N-methyldicyclohexylamine. The product also contained cis and trans-1-halo-1-alkenes, 2-halo-1-alkene, the 2,4-diene and the 1-alkene.
Still other methods for the synthesis of alkadienes have previously been reported.
Babayan, Vartanyan and Zurabov, Journal of General Chemistry of the USSR ; (in English Translation), 1955, Vol. 25, No. 8, pages 1567-1570 indicate that treatment of quaternary ammonium compounds containing the 3-chlorobutenyl radical with aqueous NaOH yields chloroprene.
Babayan, Mkrian and Giuli-Kevkhian, ibid, 1958, Vol. 28, No. 5, pages 1314-1317, report the preparation of isoprene by alkaline cleavage of quaternary ammonium salts obtained by reaction of .alpha.,.beta.- and .gamma.,.gamma.-dimethylallyl chlorides with tertiary amines. Hot aqueous NaOH was used to effect the cleavage reaction.
In the same journal, 1961, Vol. 31, No. 2, pages 752-757, Babayan and Martirosyan again refer to alkaline cleavage of quaternary ammonium salts containing 3-chlorobuten-2-yl radicals to form chloroprene and vinylacetylene, and to the formation of isoprene by alkaline cleavage of quaternary ammonium salts containing the 3-methylbuten-2-yl radical (.gamma.,.gamma.-dimethylallyl radical).
In an ensuing paper, ibid, 1961, Vol. 31, No. 2, pages 758-761, Babayan and Martirosyan report on a study, inter alia, of the effect of a methyl group in the .gamma.-position of various haloalkyl-containing quaternary ammonium salts treated with alcoholic alkali.
And, in the same journal, 1961, Vol. 31, No. 2, pages 562-566, Babayan, Gegelyan and Indzhikyan show that the introduction of methyoxymethyl substituents in the .delta.-position to .beta.,.gamma.-unsaturated groups of quaternary ammonium salts facilitates the reaction of their splitting under the influence of aqueous alkali. As Babayan, Indzhikyan, Grigoryan and Minasyan, ibid, 963. Vol. 33, No. 5, pages 1720-1726, point out, those results agree with the idea that the step which determines the rate of the reaction of alkaline splitting of quaternary ammonium salts with .beta.,.gamma.-unsaturated groups is the removal of a proton from the position to the nitrogen. Accordingly, the 1963 paper reports the results of an extension of this idea --positioning of a phenyl or para-substituted phenyl group in the .delta.-position to the nitrogen in the .beta.,.gamma.-unsaturated groups in order to take advantage of the "great protonizing action of the phenyl group".
In the case of the Babayan et al. process for producing isoprene from a quaternary ammonium salt containing an .gamma.,.gamma.-dimethylallyl group (i.e., a 3-methylbuten-2-yl group), the reactant contains no less than six extractable protons (two methyl groups) in the 4-position relative to the nitrogen atom.